U.S. patent number 6,008,915 [Application Number 08/733,591] was granted by the patent office on 1999-12-28 for method of identifying faults in wdm optical networks.
This patent grant is currently assigned to Lucent Technologies, Inc.. Invention is credited to John Lehrer Zyskind.
United States Patent |
6,008,915 |
Zyskind |
December 28, 1999 |
**Please see images for:
( Certificate of Correction ) ** |
Method of identifying faults in WDM optical networks
Abstract
In a WDM network employing optical amplifiers, a method of
detecting changes in the number of channels present in erbium doped
fiber amplifiers (EDFA) that are caused by faults or system
reconfigurations. In accordance with the technique of the present
invention, the power of one signal channel and the power of the
amplified spontaneous emission generated by the local EDFA's are
monitored.
Inventors: |
Zyskind; John Lehrer
(Shrewsbury, NJ) |
Assignee: |
Lucent Technologies, Inc.
(Murray Hill, NJ)
|
Family
ID: |
26697459 |
Appl.
No.: |
08/733,591 |
Filed: |
October 18, 1996 |
Current U.S.
Class: |
398/34; 398/177;
398/18; 398/37 |
Current CPC
Class: |
H04B
10/077 (20130101); H04B 10/07955 (20130101); H04J
14/0221 (20130101); H04B 10/296 (20130101); H04B
10/2912 (20130101) |
Current International
Class: |
H04B
10/08 (20060101); H04B 10/17 (20060101); H04B
010/02 () |
Field of
Search: |
;359/110,124-134,174-179,333-349 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lefkowitz; Edward
Attorney, Agent or Firm: Dinicola; Brian K. Brosemer;
Jeffery J.
Government Interests
ACKNOWLEDGEMENT OF GOVERNMENTAL RIGHTS
This invention was made with Government support under Agreement No.
MDA 972-94-3-0036 awarded by ARPA. The Government has certain
rights in the invention.
Parent Case Text
CROSS-REFERENCE TO RELATED PROVISIONAL APPLICATION
This application claims the benefit of the Feb. 16, 1996, filing
date of Provisional Application Ser. No. 60/023,665 entitled
"Method Of Identifying Faults In WDM Optical Networks."
Claims
What is claimed is:
1. A method of operating an optical communication network including
an optical communication path having an upstream end and a
downstream end and an optical amplifier interposed between the
upstream end and the downstream end along the communication path,
the method comprising the steps of:
transmitting a wavelength division multiplexed optical signal
between said upstream end and said downstream end, said multiplexed
optical signal having a plurality of optical channels;
receiving said wavelength division multiplexed optical signal at
said optical amplifier;
monitoring, in a first monitoring step, at an output of said
optical amplifier, changes in signal power in a first of said
plurality of optical channels;
monitoring, in a second monitoring step, at the output of said
optical amplifier, changes in amplified spontaneous emission (ASE)
power generated in an ASE band by said optical amplifier; and
determining whether one or more of the plurality of optical
channels were added or dropped at the upstream end, or
alternatively, whether signal loss was increased or decreased at
the upstream end, solely on the basis of the changes to signal
power in the first of said plurality of channels and the changes to
ASE power generated by said optical amplifier.
2. The method of claim 1, wherein the optical amplifier is an
erbium-doped fiber amplifier.
3. The method of claim 1, wherein said second monitoring step is
performed by bandpass filtering the optical signal.
4. The method of claim 1, wherein at least one of said plurality of
optical channels is modulated with digital data.
5. The method of claim 4, further comprising a step of receiving
said multiplexed optical signal at said downstream end.
6. The method of claim 1, wherein the dropping of one or more
channels upstream is identified in said determining step when the
ASE power increases and the first channel signal power increases or
drops to a null value.
7. The method of claim 1, wherein the adding of one or more
channels upstream is identified in said determining step when the
ASE power decreases and the first channel signal power decreases to
other than a null value.
8. The method of claim 1, wherein an increase in upstream signal
loss is identified in said determining step when the ASE power
increases and the first channel signal power decreases to other
than a null value.
9. The method of claim 1, wherein a decrease in upstream signal
loss is identified in said determining step when the ASE power
decreases and the first channel signal power increases.
10. The method of claim 1, wherein said first monitoring step is
performed by bandpass filtering the optical signal.
11. The method of claim 1, further comprising the step of placing a
monitor tap at the output end of the optical amplifier for
monitoring the changes to first channel signal power and the
changes to ASE power generated by said optical amplifier.
12. The method of claim 1, further comprising the step of
interposing a mid-stage filter between the upstream end and the
optical amplifier, said mid-stage filter being operative to block
power in the ASE band in the upstream WDM optical signal from being
input to the optical amplifier.
13. A system for monitoring a plurality of optical channels in a
wavelength division multiplexed (WDM) optical signal present in an
optical communication path of a communication network, the
communication path having an upstream end and a downstream end, the
system comprising:
an optical amplifier interposed between the upstream end and the
downstream end of the communication path;
a monitor tap, placed at an output end of the optical amplifier to
monitor one or more components of the optical signal;
a mid-stage filter interposed between the upstream end and the
optical amplifier, said mid-stage filter being operative to block
an amplified spontaneous emission (ASE) band in the upstream WDM
optical signal from being input to the optical amplifier; and
one or more bandpass filters interconnected to said monitor tap,
said filters being capable of filtering from the output of the
optical amplifier one of the plurality of optical channels in the
WDM signal and a signal generated by the optical amplifier in the
ASE band of the WDM signal.
14. The system of claim 13, wherein the optical amplifier is an
erbium-doped fiber amplifier.
15. The system of claim 13, wherein the monitor tap is a ten
percent tap.
16. The system of claim 13, wherein said one or more bandpass
filters pass signals associated with an ASE band substantially
centered at 1525.9 nanometers with a full wave half maximum of
approximately one nanometer.
Description
FIELD OF THE INVENTION
The present invention relates generally to optical fiber
communication networks and, more particularly, to systems and
methods for monitoring the performance characteristics of fiber
links employed in such networks.
BACKGROUND OF THE INVENTION
In WDM optical networks, which are based on the optical
transparency offered by erbium doped fiber amplifiers (EDFA's) a
number of signal channels can be transmitted simultaneously and
routed independently. In such a network, it is desirable to ensure
that when channels are added or dropped, which can result from
changes in provisioning or network reconfigurations, as well as
from network faults, the performance of the surviving channels will
be not impaired. Because of the saturation characteristics of
EDFA's this requires readjusting the EDFA gains to maintain channel
power levels within acceptable limits.
Changes in provisioning and/or network reconfiguration can give
rise to added or dropped channels, but these changes can in
principle be predicted and corrected for by the network control and
management (NC&M) system. However, it remains necessary to
detect conditions arising from system faults because these cannot
be controlled or predicted by the NC&M system. Such faults are
most likely to result from channel failures or degradation in the
optical transmission path. When these two problems happen, it is
critical they be detected so that proper measures can be taken.
Monitoring the power of only one channel or the total power of all
the channels does not provide sufficient information to determine
the appropriate corrections. Variations of upstream optical losses
or the output powers of upstream amplifiers which may occur cannot
be distinguished from dropped/added channels but call for very
different corrective actions. The proper way is to determine the
number of channels present. One possibility is to demultiplex the
channels (either in the transmission path or after a tap) and
detect the presence of each channel independently. However, this
method is complicated and expensive.
SUMMARY OF THE INVENTION
In accordance with the present invention, the aforementioned
deficiencies are overcome and an advance is made in the art by a
method in which changes in the number of channels and changes in
power common to all of the channels are monitored.
Such detection is possible because the signal power transmitted
through the system and the amplified spontaneous emission (ASE)
power generated by the local EDFA behave similarly when the number
of channels is changed but differently when the upstream loss
varies. That is, if the upstream loss changes, the changes in power
of the monitor channel and the ASE are in opposite directions. If
one or more channels are added or dropped, the changes in power
experienced by the monitor channel and the ASE are in the same
direction.
BRIEF DESCRIPTION OF THE DRAWINGS
The above features and advantages of the present invention will
become apparent from the ensuing description of several preferred
exemplary embodiments, which should be read in conjunction with the
accompanying drawings, in which:
FIG. 1 depicts an illustrative optical network employing a first
distributed feedback (DFB) laser as a multiple signal channel
source and a second DFB laser as a monitor channel source in
accordance with the present invention;
FIG. 2 is a graphical representation depicting the measurements of
amplified spontaneous emission (ASE) in the illustrative system and
multiple channels of FIG. 1 when the number of channels is
decreased; and
FIG. 3 is a graphical representation depicting the measurements of
ASE power in the illustrative system of FIG. 1 when the total input
power is decreased.
DETAILED DESCRIPTION OF THE INVENTION
An illustrative WDM optical network 10 is shown schematically in
FIG. 1. As seen in FIG. 1, a first DFB laser 12 operating at 1552.6
nm serves as the source for the monitor channel, while a second DFB
laser 14, operating at 1557.8 nm, serves to simulate the other
signal channels. Two mid-amplifier pumped, two stage EDFA's [16,
18] with dual 980 nm pumps were used to amplify the light from DFB
lasers so that suitable amount of input powers could be obtained in
each channel. The input powers were set corresponding to a network
with eight signal channels each having a power of -3.5 dBm at the
input end of the illustrative EDFA 20 which was of similar design
to the two input EDFA's. The gain of the EDFA 20 was set to be 10.5
dB by adjusting the pump power. When all eight channels were
present, the total input power was 5.5 dBm and the total output
power was 16 dBm.
A 10% tap 22 was placed at the output end of the EDFA to monitor
the output power for analysis. In the illustrative configuration of
FIG. 1, the monitor channel and ASE were selected by a bandpass WDM
24 followed by filters 26, 28. In order to measure ASE accurately,
two filters were used to pass a band of ASE centered around 1525.9
nm with a full wave half maximum (FWHM) of about 1 nm. A mid-stage
filter in the EDFA blocked the ASE generated by the upstream EDFA's
in this region. Accordingly, the ASE power measured has generated
only in the second stage of the EDFA.
The powers observed in the monitoring system when channels are
dropped are plotted in FIG. 2. When one channel is dropped, both
the power in the monitor channel and the power in the ASE channel
increased by about 0.5 dB due to the higher gain in the amplifier.
This change is large enough to permit practical detection. If more
channels are dropped, more of the amplifier's output power is
available to be redistributed to fewer surviving channels. This
means that the power increase is larger for each surviving channel,
and the contrast for detecting the number of channels is better.
The power change in the monitor channel is larger than that of the
ASE because the signal in the monitor channel goes through both
stages of the EDFA while the ASE is generated primarily in the
second stage.
In the illustrative configuration of FIG. 1, in which two band-pass
filters were employed as discussed above, the measured ASE power
was lower than it would be in a practical system. Still, this power
is deemed sufficiently large to illustrate the inventive method.
The ASE power is, for example, expected to be improved by at least
3 dB with one single narrow band filter centered around the
wavelength region that is blocked by the mid-stage filter of the
two-stage EDFA.
FIG. 3 shows the powers observed in the monitoring system when the
total input power is decreased. When the upstream loss is
increased, the power of the monitor channel decreases because of
the lower input power while the ASE power increases because of the
lower saturation and therefore higher gain. If the loss is
increased by 3 dB, the power in the monitor channel is decreased by
about 0.5 dB, and the power in the ASE channel is increased by more
than 2 dB. Thus this case, in which the changes in monitor channel
and ASE power have opposite signs, can be distinguished from the
case of dropped channels when they change with same sign.
As will be readily appreciated by those skilled in the art, the
inventive method discussed herein presumes the availability of a
suitable monitor channel. In some optical networks, one channel is
reserved for internal use by NC&M. In such networks, this
channel is always present, making it the ideal choice as the
monitor channel. For networks where no such preferred channel
exists, of course, one of the signal channels can be chosen as the
monitor channel. In case this original monitor signal channel is
dropped, however, the monitor channel must be switched to one of
the surviving channels.
The five cases for upstream loss and signal channel are tabulated
in Table 1.
TABLE 1 ______________________________________ monitor channel
power ASE power ______________________________________ upstream
loss .uparw. .dwnarw. .uparw. upstream loss .dwnarw. .uparw.
.dwnarw. number of other channels .uparw. .dwnarw. .dwnarw. number
of other channels .dwnarw. .uparw. .uparw. drop of the monitor
channel 0 .uparw. ______________________________________
From the foregoing, it will be readily appreciated by those skilled
in the art that the method of the present invention permits network
maintenance personnel to monitor the number of channels present at
any amplifier in a WDM network. Such monitoring permits effective
adjustment of the amplifier gain, for example, by adjusting the
second stage pump power, in order to optimize the performance of
surviving channels.
From the foregoing, it should be readily ascertained that the
invention is not limited by the embodiments described above which
are presented as examples only but may be modified in various ways
within the intended scope of protection as defined by the appended
patent claims.
* * * * *